It all comes down to the tradeoff between gameplay and bling. You have a finite amount of resources (much less than we have today back in 2007). Ubi stretched the capabilities of contemporary computers with what they did. What you want, a completely detailed out interior in 3D, which you could walk through, to no improvement in gameplay, would have killed SH4, rendering it nothing but a television show.
SH5 paid terrible penalties for their walkthrough capabilities, leaving you with only one submarine and half a war.
Even today, making mods, we still have to contend with the old gameplay vs bling conundrum of old, although we can be unbelievably lavish without harm. In FOTRSU, we've more than doubled the number of ships in the game, replaced the entire sound system, retextured just about everything, for instance, we're in the middle of retexturing propellers from a 128x128 texture to a 1024x1024 texture. Ducimus would shoot us. We've just increased the resources to render each propeller, completely irrelevant in gameplay, by 64 times!
Because our modern computers have unimaginable resources and memory to those 10 years ago we can get away with such frivolities, and they work well. But we still couldn't have a 3D walkthrough submarine interior for five different kinds of fleet boats and be able to keep the game enhancements we've added to the game. Our mod is 4.7 GB! I remember when we were trying to keep it under 4. When a mod is bigger than the game it modifies, then you are stretching the envelope of playability. A perfect mod that only 10% of the community can play is a failure in my mind. Ducimus would shoot us as it is.
